The system and method described herein relates generally to blade repair. More specifically, the system and method relate to an induction heating coil configured to provide uniform localized heating to the platform slash face area of the blade while maintaining visibility during a welding operation.
A gas turbine engine draws in and compresses air with an axial flow compressor, mixes the compressed air with fuel, burns the mixture, and expels the combustion product through an axial flow turbine section that powers the compressor. The turbine section of the engine includes one or more disks, each disk including a plurality of blades projecting from its periphery. The hot exhaust gases strike the blades causing the disk(s) to rotate. The rotating disk(s) are attached to a shaft that also drives the compressor. The compressor is also made from rotating disks, each disk having a plurality of blades projecting from its periphery. The disk turns rapidly on a shaft as the shaft is rotated by the turbine, and the curved blades draw in and compress air in somewhat the same manner as an electric fan.
The turbine blades on the disk are in the hot exhaust gases resulting from the combustion of the fuel and rotate at very high speeds. Thus the blades operate in an oxidative and corrosive environment, and are subjected to high operating stresses. In order to survive these harsh conditions, the turbine blades are made from superalloys, an expensive blend of elements that provide oxidation resistance, corrosion resistance and strength. These superalloys are further strengthened in preferred directions by various mechanisms which include growing the turbine blades as directional grains or even as single crystals.
The superalloys used for turbine blades include nickel-based superalloys, iron-based superalloys and cobalt-based superalloys. These superalloys can be further strengthened by precipitation mechanisms. For example, gamma prime (γ′) phases comprising Ni3Al are precipitated in the gamma matrix of the FCC crystal structure of the alloy by appropriate solutioning and aging treatments. Controlling the gamma prime phases, both the size and distribution for these as-cast, new parts is well-known. The turbine blade comprises an airfoil portion that extends into a hot gas stream, a dovetail portion that attaches the blade to the turbine disk and a platform portion that separates the airfoil portion from the dovetail portion. A shank portion is intermediate the platform portion and the dovetail portion. The turbine blades are also provided with environmental coatings and/or thermal barrier coatings to further improve their survivability in the hot, corrosive, oxidative environment of a turbine engine.
The turbine blades nevertheless are subject to damage as a result of operation in the gas turbine engine. This damage can be both mechanical in nature as well as metallurgical in nature. The turbine blades are expensive to produce, so that it is desirable from an economic standpoint to repair the blades rather than replace them whenever possible. In many situations, the blades can be repaired by removing any remaining protective coatings, followed by welding damaged mechanical areas and reworking the weld repaired areas to restore the dimensions as required, followed by reapplication of the protective coatings.
Induction heating and welding has been used to repair blades, but this method is unsatisfactory when attempting to repair the platform and slash face areas of turbine blades. Previously known induction heating coils do not provide a uniform local temperature profile in the platform and slash face regions. Non-uniform temperatures experienced by the platform and slash face regions may cause the repairs to crack and fail. In addition, the previously known induction coils obstructed the view of the welding technician during welding. Poor weld quality can result when the welding technician cannot see the entire area that needs repair, because the weld may not be properly placed or applied in the proper amount or thickness.
What is needed is a system and method that permits a uniform local temperature profile in the platform and slash face regions during weld repair of super-alloy turbine blades, while maintaining visibility of the weld area for the welding technician.